runtime/coverage: remove uses of //go:linkname

Move code to internal/coverage/cfile, making it possible to
access directly from testing/internal/testdeps, so that we can
avoid needing //go:linkname hacks.

For #67401.

Change-Id: I10b23a9970164afd2165e718ef3b2d9e86783883
Reviewed-on: https://go-review.googlesource.com/c/go/+/585820
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/internal/coverage/cfile/apis.go

@@ -0,0 +1,153 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cfile
+
+import (
+	"fmt"
+	"internal/coverage"
+	"io"
+	"sync/atomic"
+	"unsafe"
+)
+
+// WriteMetaDir implements [runtime/coverage.WriteMetaDir].
+func WriteMetaDir(dir string) error {
+	if !finalHashComputed {
+		return fmt.Errorf("error: no meta-data available (binary not built with -cover?)")
+	}
+	return emitMetaDataToDirectory(dir, getCovMetaList())
+}
+
+// WriteMeta implements [runtime/coverage.WriteMeta].
+func WriteMeta(w io.Writer) error {
+	if w == nil {
+		return fmt.Errorf("error: nil writer in WriteMeta")
+	}
+	if !finalHashComputed {
+		return fmt.Errorf("error: no meta-data available (binary not built with -cover?)")
+	}
+	ml := getCovMetaList()
+	return writeMetaData(w, ml, cmode, cgran, finalHash)
+}
+
+// WriteCountersDir implements [runtime/coverage.WriteCountersDir].
+func WriteCountersDir(dir string) error {
+	if cmode != coverage.CtrModeAtomic {
+		return fmt.Errorf("WriteCountersDir invoked for program built with -covermode=%s (please use -covermode=atomic)", cmode.String())
+	}
+	return emitCounterDataToDirectory(dir)
+}
+
+// WriteCounters implements [runtime/coverage.WriteCounters].
+func WriteCounters(w io.Writer) error {
+	if w == nil {
+		return fmt.Errorf("error: nil writer in WriteCounters")
+	}
+	if cmode != coverage.CtrModeAtomic {
+		return fmt.Errorf("WriteCounters invoked for program built with -covermode=%s (please use -covermode=atomic)", cmode.String())
+	}
+	// Ask the runtime for the list of coverage counter symbols.
+	cl := getCovCounterList()
+	if len(cl) == 0 {
+		return fmt.Errorf("program not built with -cover")
+	}
+	if !finalHashComputed {
+		return fmt.Errorf("meta-data not written yet, unable to write counter data")
+	}
+
+	pm := getCovPkgMap()
+	s := &emitState{
+		counterlist: cl,
+		pkgmap:      pm,
+	}
+	return s.emitCounterDataToWriter(w)
+}
+
+// ClearCounters implements [runtime/coverage.ClearCounters].
+func ClearCounters() error {
+	cl := getCovCounterList()
+	if len(cl) == 0 {
+		return fmt.Errorf("program not built with -cover")
+	}
+	if cmode != coverage.CtrModeAtomic {
+		return fmt.Errorf("ClearCounters invoked for program built with -covermode=%s (please use -covermode=atomic)", cmode.String())
+	}
+
+	// Implementation note: this function would be faster and simpler
+	// if we could just zero out the entire counter array, but for the
+	// moment we go through and zero out just the slots in the array
+	// corresponding to the counter values. We do this to avoid the
+	// following bad scenario: suppose that a user builds their Go
+	// program with "-cover", and that program has a function (call it
+	// main.XYZ) that invokes ClearCounters:
+	//
+	//     func XYZ() {
+	//       ... do some stuff ...
+	//       coverage.ClearCounters()
+	//       if someCondition {   <<--- HERE
+	//         ...
+	//       }
+	//     }
+	//
+	// At the point where ClearCounters executes, main.XYZ has not yet
+	// finished running, thus as soon as the call returns the line
+	// marked "HERE" above will trigger the writing of a non-zero
+	// value into main.XYZ's counter slab. However since we've just
+	// finished clearing the entire counter segment, we will have lost
+	// the values in the prolog portion of main.XYZ's counter slab
+	// (nctrs, pkgid, funcid). This means that later on at the end of
+	// program execution as we walk through the entire counter array
+	// for the program looking for executed functions, we'll zoom past
+	// main.XYZ's prolog (which was zero'd) and hit the non-zero
+	// counter value corresponding to the "HERE" block, which will
+	// then be interpreted as the start of another live function.
+	// Things will go downhill from there.
+	//
+	// This same scenario is also a potential risk if the program is
+	// running on an architecture that permits reordering of
+	// writes/stores, since the inconsistency described above could
+	// arise here. Example scenario:
+	//
+	//     func ABC() {
+	//       ...                    // prolog
+	//       if alwaysTrue() {
+	//         XYZ()                // counter update here
+	//       }
+	//     }
+	//
+	// In the instrumented version of ABC, the prolog of the function
+	// will contain a series of stores to the initial portion of the
+	// counter array to write number-of-counters, pkgid, funcid. Later
+	// in the function there is also a store to increment a counter
+	// for the block containing the call to XYZ(). If the CPU is
+	// allowed to reorder stores and decides to issue the XYZ store
+	// before the prolog stores, this could be observable as an
+	// inconsistency similar to the one above. Hence the requirement
+	// for atomic counter mode: according to package atomic docs,
+	// "...operations that happen in a specific order on one thread,
+	// will always be observed to happen in exactly that order by
+	// another thread". Thus we can be sure that there will be no
+	// inconsistency when reading the counter array from the thread
+	// running ClearCounters.
+
+	for _, c := range cl {
+		sd := unsafe.Slice((*atomic.Uint32)(unsafe.Pointer(c.Counters)), int(c.Len))
+		for i := 0; i < len(sd); i++ {
+			// Skip ahead until the next non-zero value.
+			sdi := sd[i].Load()
+			if sdi == 0 {
+				continue
+			}
+			// We found a function that was executed; clear its counters.
+			nCtrs := sdi
+			for j := 0; j < int(nCtrs); j++ {
+				sd[i+coverage.FirstCtrOffset+j].Store(0)
+			}
+			// Move to next function.
+			i += coverage.FirstCtrOffset + int(nCtrs) - 1
+		}
+	}
+	return nil
+}